The Phase Diagram

Generally, heating a solid will cause it to melt to a liquid (at the melting or
freezing point). Continued heating (to the boiling point) produces a gas. If you follow
the horizontal line beginning at the pressure axis at 1 atm, the substance begins as a
solid, passes the normal freezing (or melting) point into a liquid, then boils to a gas.
For most substances, at higher pressures, the freezing and boiling points are higher.
Once boiling begins the temperaturer remains fixed at the boiling point until all of the
liquid is converted to gas. Thus, at the boiling point liquid and gas coexist in
equilibrium. Similarly, solid and liquid coexist in equilibrium at the melting or boiling
point.

The triple point is the temperature and pressure at which all three
phases can exist in equilibrium.
Above the critical point, molecules are unable to liquify.

The heat of fusion is the amount of energy given off when a substance
freezes (or the amount of energy the substances requires to liquify).
The heat of vaporization is the amount of energy given off when a gas
liquifies (or the amount of energy needed to vaporize the liquid).